1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method, and relates in particular to a semiconductor device and a manufacturing method where external leads are welded to the electrodes of a semiconductor chip.
2. Description of Related Art
Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) utilized in power supplies for consumer electronic products such as cell phones, personal computers, and digital audio-visual equipment or for driving vehicle motors carry a large current of approximately 1 to 200 A. This heavy current requires using a thick external wire with a large cross sectional area. Therefore in order to draw a large current, multiple large gage bonding wires of about 100 to 500 μm are connected as external wires to a MOSFET source electrode.
Bonding wires are typically connected by ultrasonic bonding or thermocompression bonding to the source electrode. This bonding connection requires an area of about 1.5 to 3 times the cross sectional area of the wire. The source electrode or gate electrode however is limited to a small area so there is a limit on how far the wire cross sectional area can be enlarged. Moreover the large wire cross sectional wire requires a higher application of pressure to obtain the required larger bonding strength and that mechanical impact causes damage to the semiconductor chip. The loop in the large-gage bonding wires also causes lower productivity.
To address these problems, the Japanese Unexamined Patent Application Publication Nos. 2002-313851 (Oono et al.) and 2002-314018 (Funato et al.) utilize a flat-plate electric member as an external wire instead of a bonding wire and to enlarge the cross sectional area of the external wire. FIG. 1 is an external perspective view of the SOP (Small Outline Package) disclosed in Ono et al. FIG. 2A is a horizontal cross sectional view taken along lines A-A of FIG. 1. FIG. 2B is a vertical cross sectional view taken along lines B-B of FIG. 1.
The semiconductor device 101 in FIGS. 2A and 2B includes a molding resin 102. A source electrode 104a and a gate electrode 104g are forming on the top surface of the semiconductor chip 105, and a drain electrode (not shown) is formed on the underside of semiconductor chip 105.
Among the eight external leads, four drain-side terminals 103d are integrated into one set to form drain-side post 107d inside the molding resin 102. The semiconductor chip 105 is placed above the drain-side post 107d such that the drain-side post 107d is electrically connected with the drain electrode (not shown). Among the remaining four external leads, three source-side terminals 103s are integrated into one set to form a source-side post 107s within the molding resin 102, and one gate-side terminal 103g forms a gate-side post 107g within the molding resin 102.
The source-side post 107s and the source electrode 104s are electrically connected by an electric path member 106. An electrode-side connecting portion 106a formed at one end of the electric path member 106 and source electrode 104s; and a lead-side connecting portion 106d formed at the other end of the electric path member 106 and source side post 107s are connected directly to each other by ultrasonic bonding. Utilizing the flat plate electric path member 106 allows making the cross sectional area of the path for current flowing between the source-side post 107s and the source electrode 104s significantly larger than the path for current flowing through the multiple bonding wires. The gate-side post 107g is electrically connected by one bonding wire 108 to the gate electrode 104g. 
A bonding technique for wire bonding utilizing a laser is disclosed in Japanese Unexamined Patent Application Publication No. SH061-53737 (FIG. 2) (Matsuda et al). FIG. 3 is a cross sectional view showing a semiconductor device disclosed in Matsuda et al. A semiconductor chip 201 is placed on a die pad (islands) 203 of a lead frame. An electrode pad 202 placed on the semiconductor chip 201 and a lead 204 of the lead frame are electrically connected by a bonding wire 205. Laser welding connects the electrode pad 202 and the wire 205.
However, the ultrasonic bonding process disclosed by Oono et al. and Funato et al. may cause mechanical damage to the semiconductor chip due to ultrasonic vibration. Moreover bonding to some metals is difficult because the process is a mechanical bonding process.
Specifically the step of mounting a semiconductor chip on a die pad (island) of a lead frame is carrier out at a temperature of 300 degrees C. or higher and an oxide film is consequently formed on the surface of the semiconductor chip electrode. In order to make a mechanical bond between the external wire and the electrode at a low temperature, the oxide film on the electrode surface must be stripped away to expose a new surface. These actions require applying an ultrasonic vibration to the external wire and the electrode that were placed in contact with each other. However if the external wire has a large cross sectional area then a large mechanical impact is applied to the electrode due vibration of the external wire, and convey the mechanical damage to the semiconductor chip under the electrode. The MOSFET product is a product especially vulnerable to breakdown since there is an active cell directly under the electrode. Moreover, the bonding is difficult when there is a material such as copper (Cu) with a thick surface oxide film.
The laser bonding process disclosed by Masuda et al. on the other hand, might cause thermal damage to the semiconductor device under the electrode. Large diameter wires of about 100 to 500 μm or more are used to lower the on-resistance to allow the MOSFET to carry larger current. This current corresponds to a thickness of 100 to 500 μm. The thickness of the electrodes on other hand may be as small as 2 to 6 μm as described in Oono et al. Adjusting the laser intensity is extremely difficult when laser-welding members having significantly different thicknesses because a high laser intensity might thermally damage the semiconductor device, and a low laser intensity might fail to make the connection or make only a weak connection.